Since their appearances in 1991 as a small, lightweight and high-capacity battery, lithium secondary batteries have been widely used as a power source of portable devices. With recent rapid developments in electronics, communications and computer industries, camcorders, mobile phones, laptops, PCs and the like have appeared and gone through remarkable developments, and demands for secondary batteries as an energy source driving these portable electronics and information communication devices have continuously increased.
However, lithium secondary batteries have a problem in that battery lifespan rapidly decreases as charge and discharge are repeated. Such a problem is particularly more serious at high temperatures and high voltages. This is due to a phenomenon occurring when an electrolyte is decomposed or an active material is degraded due to moisture inside the battery or other influences, or inner resistance of the battery increases.
In view of the above, a positive electrode active material for a lithium secondary battery that has been actively research and developed, and is currently used is a layer-structured LiCoO2. LiCoO2 is most widely used due to its excellent lifespan property and charge and discharge efficiency, but has low structural stability, and therefore, has a limit in the use in technologies enabling batteries to have high capacity.
Various lithium transition metal oxides such as LiNiO2, LiMnO2, LiMn2O4, LiFePO4 or Li(NixCoyMnz)O2 have been developed as an alternative positive electrode active material. Among these, LiNiO2 has an advantage of exhibiting a battery property of high discharge capacity, but has a problem of being difficult to be synthesized using a simple solid state reaction, and having poor thermal stability and cycle property. In addition, lithium manganese-based oxides such as LiMnO2 or LiMn2O4 have an advantage of excellent thermal stability and low costs, but have a problem of small capacity and a poor high temperature property. Particularly, some LiMn2O4 has been commercialized as low-priced products, but does not have a favorable lifespan property due to structural distortion (Jahn-Teller distortion) caused by Mn3+. Furthermore, extensive studies have been made on LiFePO4 for application in hybrid electric vehicles (HEV) due to its low costs and excellent stability, however, LiFePO4 is difficult to be used in other fields due to its low conductivity.
Under such circumstances, a material mostly favored as an alternative positive electrode active material for LiCoO2 is Li(NixCoyMnz)O2 (herein, x, y and z are each independently an atomic fraction of oxide-forming elements, and 0<x≤1, 0<y≤1, 0<z≤1 and 0<x+y+z≤1). This material is less expensive than LiCoO2, and has an advantage of capable of being used under high capacity and high voltage, but has a disadvantage of inferior rate capability and lifespan property at high temperatures.
Accordingly, various attempts to improve thermal stability, a capacity property, a cycle property or the like of a positive electrode active material through methods such as doping materials such as Al, Ti, Sn, Ag or Zn into the positive electrode active material, or dry or wet coating metals having favorable conductivity on a surface of the positive electrode active material have been made, however, the extent of improvement is still insufficient.
Particularly, when a positive electrode active material is doped, the doped materials are present in a uniform concentration in the positive electrode active material, which causes a problem of capacity decline although structural stability of the positive electrode active material is enhanced.